The present invention relates to processes for scrambling television signals transmitted over the air or via a cable environment, and in particular to a scrambling technique which generates non-interlaced horizontal sync modulation as well as a double horizontal pulse in selected horizontal blanking intervals, to enhance horizontal tearing and vertical instabilities in a program video picture. The invention is particularly effective over prior art scrambling systems which use typical horizontal and vertical overlay signals, when applied to modern television set scanning systems using count down circuits with ceramic resonator voltage controlled oscillators. The invention""s effectiveness is not limited to television sets and monitors using cathode ray tube displays. It is effective in other display systems such as LCD, plasma display and other solid state display systems.
In typical prior art scrambling systems, the horizontal and/or vertical sync modification techniques are ineffective on more modern television sets. With their scanning systems containing digital circuits, such modern sets can lock to a modified television signal with or without vertical sync pulses. As a result, a scrambled television signal containing prior art time position varying vertical sync signals has no effect on such modern television sets, even though such prior art signals will induce good concealment for scrambling purposes on older sets using conventional horizontal oscillators.
The prior art horizontal overlay sync modifications of previous mention use an interleaved pattern. This interleaved (interlaced) pattern in a field-to-field basis was thought to achieve maximum concealment, however such an interleaved (interlaced) pattern is found to impart less concealment because the tearing is distributed over two fields. As a result, the tearing appears to be diluted.
In contrast, in the present invention, more scrambling concealment is achieved since the horizontal sync is position modulated at a non-interleaved frequency that is a multiple of the field frequency. This consistently causes a tearing pattern which is generally the same in every field and that is less revealing of picture content.
It should be noted that in some horizontal sync width modulation schemes (i.e. the horizontal sync is pulse width modulated roughly 3:1 for 2.4 microseconds (xcexcs) to 7 xcexcs), decoding artifacts arise because the television or cable tuner detects the varying horizontal sync pulse width for its automatic RF and/or IF gain control (AGC). As a result, a narrowed horizontal sync width delivers a different AGC voltage than a wider horizontal sync width. The video gain of the tuner is then modulated according to the horizontal sync width and thus, causes undesirable AGC artifacts.
Exemplary objects and advantages of the invention are:
a) To generate unique preferably non-interlaced horizontal sync modulation for a scrambling process, that can be descrambled, which causes more effective horizontal concealment to an unauthorized viewer:
b) To generate unique multiple horizontal sync modulation which is combinable with position varying vertical sync pulses in a scrambling process, that can be descrambled, which causes more horizontal and vertical concealment when viewed by an unauthorized viewer. This increased concealment occurs especially when viewed on a more modern television set such as those sets using ceramic resonators and digital count down circuits in their scanning system; and
c) To provide a new scrambling method as described in xe2x80x9caxe2x80x9d or xe2x80x9cbxe2x80x9d above with reduced or no AGC artifacts in a television or cable tuner.
The present invention overcomes the shortcomings and problems of the aforementioned prior art while providing a method and apparatus which achieves the objects and advantages of previous mention.
To this end, a first embodiment of the present invention comprises a method and apparatus for modifying, i.e. scrambling, the horizontal sync and burst within a standard horizontal blanking interval of, for example, about 11 microseconds (xcexcs). If a subsequent descrambler system replaces such a scrambled sync and burst beyond the standard horizontal blanking interval (i.e. greater than 11 xcexcs; for example, 14 xcexcs), the modified or scrambled sync and burst occur over an extended horizontal interval and thus yield greater horizontal concealment.
In accordance with a first embodiment of the invention, horizontal sync and/or burst envelope signals preferably are narrowed by about half within the horizontal blanking interval. In one situation, the narrowed sync and/or burst are located at a position just after the end of an active program video line. This may allow for a breezeway of approximately 400 nanoseconds (ns) if desired, which is not required in the present invention (200 ns to 500 ns will also work for the breezeway period). In another location, the narrowed sync and/or burst are positioned closest to the beginning of the following program video line. The net positional displacement of these two narrowed sync and burst locations is a xe2x80x9cgapxe2x80x9d of about 6 xcexcs (even more displacement exists in the situation where the horizontal blanking interval, and thus the gap, is enlarged).
The scrambling method of the invention includes position modulating or switching between the two locations (inclusive) at a rate of N * Field Frequency where N is a positive whole number. For example, in NTSC, the field frequency is 59.94 Hertz (Hz). It has been found with many television sets, the maximum concealment (is at N=11) and the rate of modulation is a non-interleaved frequency of about 660 Hz. In this embodiment, it has been found that field-to-field horizontal overlay patterns with an interleaved frequency such as 690 Hz do not provide as much concealment as the pattern with a non-interleaved frequency of 660 Hz. The invention is equally useful in other scanning standards including PAL, SECAM and higher line and field rates used in higher definition television systems.
For slight rolling effects (up or down the screen) that can cause even more concealment, a modified rate of sync modulation for the 660 Hz example is from 654 Hz to 666 Hz. Thus, the following range of frequencies based on N=8, N=9, N=10, N=13, will also provide concealment: 474 Hz to 486 Hz, 534 Hz to 546 Hz, 594 Hz to 606 Hz, 714 Hz to 726 Hz, 774 Hz to 786 Hz, respectively, and the like. These different rate frequencies may be used in any combination in position modulating the modified horizontal sync pulse throughout the field. For example, the top and bottom third of the television field may have the modified horizontal sync pulse modulated at 714 Hz while the middle portion of the field may have the modified horizontal sync pulse modulated at 665 Hz.
Accordingly, to provide optimum horizontal concealment with a slight rolling effect vertically, the rate of sync modulation is for example made equal to N * Field Frequency plus or minus K * 10% of field frequency, where K generally is between 0 and 1 inclusive. For example, K can be larger than 1.
The preferably narrowed horizontal sync widths are about the same for each television line, regardless of location or position, therefore tuner AGC artifacts are reduced or eliminated.
In a second (alternative) embodiment, fake vertical sync (V sync) pulses are inserted before and/or after the normal vertical sync pulse""s location (for example, in NTSC, lines 1-3 and lines 18-20) and the normal vertical sync pulses (broad sync pulses) are taken out. These fake pre-vertical and post-vertical sync pulses are for example, alternately switched from one location (pre-V sync) to another (post-V sync) at a selected frequency, such as for example, field rate or at selected random rates. Normally these fake vertical pulses will cause jumping up and down (vertical jumping) of the picture in many television sets, but not in the more modern sets which use ceramic resonator oscillators and digital count down circuits in the scanning system. However, simultaneous application of the horizontal sync modifications as described in the first embodiment above does cause sufficient horizontal and/or vertical tearing and/or vertical jumping of the picture on the more modern sets.
For even more concealment when applying vertical jumping as well, small portions of the top and bottom of the active field, including the vertical blanking interval if possible, include an added horizontal sync pulse to provide a further technique of double horizontal sync modification (xe2x80x9cdouble syncxe2x80x9d). This double sync modification switches in a second horizontal sync pulse per line to cause for example the count down circuits in the modern television sets to miscount or misbehave, thus causing vertical rolling and attendant picture concealment. The small portions of the top and/or bottom and/or the vertical blanking interval, where the double sync pulses are generally switched in (either all or part of the time), does not cause as much horizontal tearing as does the first embodiment. However, this is not necessarily bad for this double sync modification scrambling process. For example, since most of the program material of interest generally is in the middle 70 to 85% of the active television field, reduced concealment in the other 15 to 30% of the television field is a small penalty as long as the middle of the field is sufficiently scrambled. It follows that a preferred modification for the second embodiment of the invention, for example in the middle 80% of the active field, also includes the horizontal sync modification as described in the first embodiment above. It should be noted that the double horizontal sync pulses of the second embodiment can be applied anywhere in the television field as well, and when combined with fake vertical pulses will cause the picture in modern sets to vertically jump and also tear horizontally.
In the second embodiment, where AGC artifacts are concerned, it is preferable to have the same effective horizontal pulse width per line when applying the double sync modification of previous mention. To achieve this, in the double sync modification, the summation of the double sync pulse widths in the small portion of lines at the top and/or bottom, and/or the vertical blanking interval locations, is substantially equal to the single horizontal sync pulse width of the other lines. When a double sync is switched in, each of the individual sync pulses therefore is narrower in width than the width of the single pre- or post-horizontal sync pulse. For example, a 4 xcexcs sync pulse is used in the areas having the small portion of lines. When double sync pulses are switched in, two 2 xcexcs horizontal sync pulses may be used per television line. However it is readily apparent that the width of each double sync pulse does not have to be exactly one half of the normal pulse. These conditions are illustrated below in the discussion of FIGS. 1E through 1G.